Molecular oxygen in model biomembranes and real biomembranes will be investigated by ESR linewidth studies of spin probes aided by NMR and direct chemical analysis. In theory, the ESR linewidth due to dissolved oxygen, should be a function of the oxygen concentration, the oxygen diffusion coefficient and the oxygen spin-lattice relaxation time. In the next year we will work to understand the relative importance of each of these variables in order to establish the general analytical utility of using spin probes to study dissolved oxygen. The concentration of oxygen and the diffusion coefficient of oxygen will be studied as a function of lipid head group and chain length, physical state of the lipids, and spin probe location within a bilayer.